Out of concern for environmental pollution by dioxins which are strongly toxic substances, it is required in various countries to analyze and evaluate contamination by dioxins with respect to exhaust gases from waste incineration facilities, the atmospheric air, water such as plant effluent or river water, fly ash generated in waste incineration facilities and soil. Such analysis and evaluation are often required also for foods.
The term “dioxins” generally encompasses polychlorinated dibenzo-para-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and dioxin-like polychlorinated biphenyls (DL-PCBs). Among 209 kinds of polychlorinated biphenyls (PCBs), DL-PCBs are PCBs showing toxicity similar to those of PCDDs and PCDFs, and include non-ortho PCBs and mono-ortho PCBs.
In evaluating contamination by dioxins for samples including environmental samples such as the atmospheric air and soil, and food samples, first, it is necessary to extract dioxins from the sample to obtain an analytical sample. When the sample is a solid such as soil or a solid food, dioxins are extracted from the solid, for example, by a Soxhlet extraction method. When the sample is a fluid such as the atmospheric air or a beverage, for example, dioxins in the fluid are trapped by using a collector such as a filter, and then the collector is washed or subjected to a Soxhlet extraction method to extract the dioxins collected thereby. The extract of dioxins obtained in accordance with the above manners is subjected as an analytical sample to quantitative analysis by using an analyzer such as a gas chromatography mass spectrometry (GC/MS).
The extract of dioxins contains various impurities that can influence on the analysis result, for example, polychlorinated polycyclic aromatic hydrocarbons resembling dioxins in chemical structures and chemical behavior such as polychlorinated diphenyl ether (PCDE) and PCBs other than DL-PCBs (hereinafter, also referred to as non-DL-PCBs). Therefore, the extract is normally concentrated as appropriate after subjected to purification, and then applied to the analyzer. As a method for purifying the extract, Patent Literature 1 discloses a method of using a chromatograph column equipped with a primary column packed with sulfuric acid silica gel and silver nitrate silica gel as purifying agents, and a secondary column packed with activated carbon-containing silica gel or graphite carbon as an adsorbing agent. In this method, as an adsorbing agent of the secondary column, activated carbon-containing silica gel or graphite carbon can be selectively used, and when both of these adsorbing agents are used, they may be used in a layered state or in a mixed state.
In the purification treatment method using this chromatograph column, first, an extract of dioxins is injected into the primary column, and then the primary column is supplied with a hydrocarbon solvent. The hydrocarbon solvent dissolves dioxins in the injected extract, and passes through the primary column and the secondary column. During the passage, dioxins dissolved in the hydrocarbon solvent pass through the purifying agent of the primary column, and are adsorbed to the adsorbing agent of the secondary column. On the other hand, impurities contained in the extract are dissolved in the hydrocarbon solvent together with dioxins, and partially degraded while they are passing through the purifying agent of the primary column, and partially adsorbed. Among the impurities or the degradation products thereof, those not adsorbed in the purifying agent pass through the adsorbing agent of the secondary column in the state of being dissolved in the hydrocarbon solvent and are then discharged from the column.
Next, the primary column and the secondary column are separated, and the secondary column is supplied with an alkylbenzene capable of dissolving dioxins. By collecting the alkylbenzene passing through the secondary column, it is possible to obtain an impurities-free alkylbenzene solution of dioxins. This alkylbenzene solution can be used as an analytical sample for dioxins, and is subjected to analysis by an analyzer such as a GC/MS after appropriate concentration.
In such a purification treatment method, all kinds of dioxins contained in the extract are adsorbed to the adsorbing agent in the secondary column, and the adsorbed dioxins are extracted with alkylbenzene. Therefore, in the analyzer, all kinds of dioxins contained in the alkylbenzene solution are analyzed at once.
However, in analysis of an alkylbenzene solution containing all kinds of dioxins at once, the obtainable result can be unreliable. For example, it is known that when the alkylbenzene solution is analyzed by a high-resolution GC/MS, mono-ortho PCBs influence on quantitative analysis results of PCDDs and PCDFs, and inversely PCDDs and PCDFs influence on quantitative analysis results of mono-ortho PCBs.
For this reason, in analysis of dioxins, an attempt has been made to prepare an analytical sample by fractionating dioxins into several kinds. For example, Patent Literature 2 discloses a method of using graphite-like carbon or a mixture of graphite-like carbon and other materials such as silica gel, activated carbon-containing silica gel, activated carbon, alumina or zeolite as an adsorbing agent for dioxins.
In this method, a column packed with an adsorbing agent is supplied with a purified dioxins solution to cause dioxins to be adsorbed to the adsorbing agent. Then, the column is sequentially supplied with several kinds of solvents to prepare several kinds of solutions of dioxins. Patent Literature 2 states that this method makes it possible to prepare, for example, three kinds of solutions of dioxins: a solution containing PCBs other than DL-PCBs, a solution containing mono-ortho PCBs, and a solution containing non-ortho PCBs, PCDDs and PCDFs.
However, in this method, it is difficult to finely fractionate dioxins because all kinds of dioxins are to be adsorbed to the adsorbing agent as is the case with the method described in Patent Literature 1. For example, part of PCDDs and PCDFs can be mixed into the solution containing mono-ortho PCBs, and part of mono-ortho PCBs can be mixed into the solution containing non-ortho PCBs, PCDDs and PCDFs.